Problem: Which of these numbers is prime? ${9,\ 13,\ 48,\ 75,\ 99}$
A prime number has exactly two factors: 1 and itself. The factors of 9 are 1, 3, and 9. The factors of 13 are 1 and 13. The factors of 48 are 1, 2, 3, 4, 6, 8, 12, 16, 24, and 48. The factors of 75 are 1, 3, 5, 15, 25, and 75. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 13 is a prime number.